Effect of scattered megavoltage x-rays on markerless tumor tracking using dual energy kilovoltage imaging

Abstract

Purpose: To determine the effect of megavoltage (MV) scatter on the accuracy of markerless tumor tracking (MTT) for lung tumors using dual energy (DE) imaging and to consider a post-processing technique to mitigate the effects of MV scatter on DE-MTT. Methods: A Varian TrueBeam linac was used to acquire a series of interleaved 60/120 kVp images of a motion phantom with simulated tumors (10 and 15 mm diameter). Two sets of consecutive high/low energy projections were acquired, with and without MV beam delivery. The MV field sizes (FS) ranged from 2 × 2 cm2–6 × 6 cm2 in steps of 1 × 1 cm2. Weighted logarithmic subtraction was performed on sequential images to produce soft-tissue images for kV only (DEkV) and kV with MV beam on (DEkV+MV). Wavelet and fast Fourier transformation filtering (wavelet-FFT) was used to remove stripe noise introduced by MV scatter in the DE images ((Figure presented.)). A template-based matching algorithm was then used to track the target on DEkV, DEkV+MV, and (Figure presented.) images. Tracking accuracy was evaluated using the tracking success rate (TSR) and mean absolute error (MAE). Results: For the 10 and 15 mm targets, the TSR for DEkV images was 98.7% and 100%, and MAE was 0.53 and 0.42 mm, respectively. For the 10 mm target, the TSR, including the effects of MV scatter, ranged from 86.5% (2 × 2 cm2) to 69.4% (6 × 6 cm2), while the MAE ranged from 2.05 mm to 4.04 mm. The application of wavelet-FFT algorithm to remove stripe noise ((Figure presented.)) resulted in TSR values of 96.9% (2 × 2 cm2) to 93.4% (6 × 6 cm2) and subsequent MAE values were 0.89 mm to 1.37 mm. Similar trends were observed for the 15 mm target. Conclusion: MV scatter significantly impacts the tracking accuracy of lung tumors using DE images. Wavelet-FFT filtering can improve the accuracy of DE-MTT during treatment.